Electromagnetic compatibility (EMC) is the study of accident electromagnetic energy in electricity generation, propagation and reception, as well as the harmful effects caused by this energy. EMC's goal is in the same environment, electromagnetic phenomena involving different devices are able to operate normally, but this is not the environment to any device the ability to produce intolerable electromagnetic interference. Traditionally said, EMC contain EMI (electromagnetic interference) and EMS (electromagnetic susceptibility) in two ways.
Electromagnetic interference (EMI) is any conduction or electromagnetic field associated with the applied voltage and current generated will reduce the performance of a device, equipment or system, or adversely affect the electromagnetic phenomena.
LED power of electromagnetic interference, the main aspects of engineers to consider are: circuit measures, EMI filtering, component selection, interference shielding and printed circuit board design.
LED power supply design for engineers, electromagnetic interference is a key issue has been present in the design. How can I solve this problem? Let's look at several factors that influence the electromagnetic compatibility.
A number of factors affect the EMC
Circuit configuration (1) of the drive power source
Initially the LED power supply is linear, but linear power at work will be a lot of energy loss in the form of heat. Linear power work, so he must have changed from high to low pressure pressure device, generally are transformer and then rectified output DC voltage. Though bulky, heat, the advantage of external interference, small electromagnetic interference, but also easy to solve.
And now more and more use of LED switching power supply, are in the form of PWM LED driver power supply is to work in the power transistor is turned on and off states. In conduction, low voltage, large current; shutdown, high voltage, current, and therefore the loss of power semiconductor devices produced is small. The disadvantage is obvious, electromagnetic interference (EMI) and more serious.
(2) switching frequency
LED power electromagnetic compatibility problem is generally the power switch circuit. The switch circuit is one of the main sources of interference switching power supply.
LED drive power switching circuit is the core of the switching circuit consists of switches and high-frequency transformer. it produces du / dt has a relatively large pulse, wide-band and rich harmonics.
The main reason is the high-frequency pulse interference: load switch high-frequency transformer primary coil is inductive load.
Turned the moment, the primary coil produces a large inrush current, and the emergence of high surge peak voltage across the primary winding; opened and the moment due to the leakage flux of the primary coil, resulting in no part of the energy transfer from the primary coil to two secondary coil, with a damped oscillation circuit formed spikes superimposed on the turn-off voltage, forming a turn-off voltage spike.
Produce more high-frequency pulse emission, the periodic signal to produce more emissions. In the LED power supply system, the switching circuit generates current spikes, and when the load current changes will generate a current spike. This is one of the sources of electromagnetic interference.
(3) Ground
In all EMC title, the main problem is caused by improper grounding. There are three signal ground methods: single-point, multi-point and mixing. When the switch circuit frequencies below 1MHz, use a single-point ground method, but not suitable for high frequency; in high frequency applications, preferably multi-point grounding. The ground is mixed with a low-single-point ground and the high frequency multi-point grounding method. Ground layout is the key, high-frequency digital circuits and grounding circuit low-level analog circuits do not mix.
(4) PCB design
Appropriate printed circuit board (PCB) routing is essential to prevent EMI.
Reset circuit design (5) Smart LED power supply
In the LED power supply, LED power supply with a lot of intelligent MCU control, and some control of single LED power switching circuit duty cycle, SCM watchdog system running the entire LED power plays a particularly important role, since All sources of interference can not be all it would be a valid defense barrier which provides error correction or addition to, once entered the CPU interfere with the normal operation of the program, then reset the system combines software action. Commonly used to reset the system has the following two:
① external reset system. External "watchdog" circuit can also be used to design their own special "watchdog" to build chips. Thus, if the system program stuck in an endless loop, and the loop has happened to "feed the dog" signal, then the reset circuit can not achieve its proper function of.
② Now more and more LED power supplies have reset the system on its own slice, so users can easily use its internal reset timer, but some intelligent LED power supply control circuit reset command is too simple, so there will still be an infinite loop as described above as "feed the dog" instruction to lose monitoring role.
Hardware Processing Method II of interference measures
To solve the problem of electromagnetic interference LED drive power is available from the following aspects:
1. reduce the switching noise on the power itself
① soft-switching technology: increase the inductance and capacitance element in the original hard switching circuit, the resonant inductor and capacitor to reduce switching process of du / dt and di / dt, so that drops the voltage switching device opened before the current up, down prior to the current rise in voltage, voltage and current to eliminate the overlap time or off.
② switching frequency modulation technique: by modulating the switching frequency fc, the focus fc and its harmonics 2fc, energy 3fc ... spread on the band around them, in order to reduce EMI amplitude of each frequency point.
Select ③ Component: Choose easy to produce noise, easy conducted and radiated noise components. Typically particular note is that the choice of diodes and transformer windings class components. Reverse recovery current, shorter recovery time fast recovery diode is ideally suited for high-frequency switching power supply rectification section.
④ rational use of EMI filters: One of the main purposes of the EMI filter, a power grid noise is electromagnetic interference, radio frequency interference it belongs (RFI), which conducted noise spectrum is substantially 10KHz ~ 30MHz, up to 150MHz.
In general, differential mode interference amplitude, low frequency interference caused by the smaller; common mode interference amplitude, high frequency, can also produce radiation through the wire, the larger the interference caused. To weaken conducted interference, the most effective way is to install the EMI filter input and output switching power supply circuit.
LED power generally use a simple single-stage EMI filters, including common mode choke and filter capacitor.
Figure 2 is a conventional LED power filter, L, C1 and C2 are used to filter out common mode interference, C3 and C4 series mode interference filtered out. When the common-mode interference occurs due to the same magnetic flux direction in the two coils L, after coupling the total inductance increases rapidly, so the common mode signal exhibits a large inductive reactance, so that is not easy to pass, so called common mode choke coil. Its two coils are wound on a ferrite toroid low loss, high magnetic permeability. R is the discharge resistance can be accumulated charge on C3 vent let go, to avoid charge accumulation affect the filter characteristic, but also to power after the end of the power line L, N is not charged, to ensure the safety of use.
⑤ EMI filter can effectively suppress electromagnetic interference Switching power adapter
FIG. 3 is a curve without EMI filter switch on the power adapter 0.15MHz ~ 30MHz conduction noise waveform.
Curve b is a waveform added after EMI filters, electromagnetic interference can be attenuated 50 db (Uv) ~ 70 db (uV). Clearly, better EMI filter insertion.
Front and rear transmission electromagnetic interference filter was added to the noise voltage across the load, respectively U1 and U2, the formula is 20lgU1 / U2.
Expressed in decibels dB insertion loss, the greater the decibel value, indicating a stronger ability to suppress noise.
Add loss measuring circuit shown in FIG. e is the noise signal generator, Zi is the internal impedance of the signal source, ZL is the load impedance, and generally 50 ohms. Optional noise frequency range 10KHz ~ 30MHz. First were measured at different frequencies of noise around U1 join EMI filter pressure drop across the load, U2, and then into the formula 20lgU1 / U2 calculated insertion loss values for each frequency, and finally export insertion loss curves.
2. cut off the route of transmission of the interfering signal
① power line disturbances can use the filter to remove the power cord. A reasonable and effective switching power supply EMI filter should supply line differential mode and common mode interference has a strong inhibitory effect.
② improve the electromagnetic compatibility design of PCB
PCB is LED power supply system support components and devices, which provide electrical components and connections between devices. With the rapid development of electronic technology, PCB density is increasing. PCB design quality great influence on electromagnetic compatibility LED power supply.
Practice has proved that, even if the circuit schematic design right, printed circuit board design properly, will adversely affect the reliability of the LED power supply.
PCB anti-jamming design includes PCB layout, wiring and grounding, which aims to reduce crosstalk and electromagnetic radiation PCB on the PCB circuit.
Also, the general transformer electromagnetic interference caused by the hum frequency is generally about 50HZ, improper wiring and grounding hum due to the multiplier effect of the frequency of the rectifier circuit is about 100HZ, carefully distinguish still perceptible.
The correct wiring method is to select the main filter capacitor pin as a centralized ground, strong and weak signal ground strictly separated, are summarized in the total ground.
Therefore, in the design of printed circuit boards, we should pay attention to the correct way to comply with the general principles of PCB design, and shall conform to the requirements of anti-jamming design.
3. The enhanced anti-jamming capability disturbed body
In the LED power supply system input / output is also the source of interference conductive lines, and receiving radio frequency interference signal source Pick up, we generally take effective measures designed to:
① adopt the necessary common-mode / differential mode suppression circuit, but also to take certain filtering and anti-electromagnetic interference shielding measures to reduce feed intake.
② take as much as possible under various conditions permit isolation measures (such as magnetic or optical isolation isolation), thereby blocking the spread of interference.
③ lightning protection measures
LED power system for outdoor use or exclusion from the introduction of indoor outdoor power lines, signal lines, to consider the system of lightning problems. Commonly used anti-lightning devices include: gas discharge tube, TVS (Transient Voltage Suppression) and the like. Gas discharge tube when the power supply voltage is greater than a certain value, typically tens or hundreds of V V, gas breakdown discharge, the strong impulse power line lead into the earth. TVS can be seen as two parallel and opposite direction Zener diode when the voltage across the conduction above a certain value. Which can be characterized by a transient current is hundreds of thousands of A's.
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Electromagnetic interference (EMI) is any conduction or electromagnetic field associated with the applied voltage and current generated will reduce the performance of a device, equipment or system, or adversely affect the electromagnetic phenomena.
LED power of electromagnetic interference, the main aspects of engineers to consider are: circuit measures, EMI filtering, component selection, interference shielding and printed circuit board design.
LED power supply design for engineers, electromagnetic interference is a key issue has been present in the design. How can I solve this problem? Let's look at several factors that influence the electromagnetic compatibility.
A number of factors affect the EMC
Circuit configuration (1) of the drive power source
Initially the LED power supply is linear, but linear power at work will be a lot of energy loss in the form of heat. Linear power work, so he must have changed from high to low pressure pressure device, generally are transformer and then rectified output DC voltage. Though bulky, heat, the advantage of external interference, small electromagnetic interference, but also easy to solve.
And now more and more use of LED switching power supply, are in the form of PWM LED driver power supply is to work in the power transistor is turned on and off states. In conduction, low voltage, large current; shutdown, high voltage, current, and therefore the loss of power semiconductor devices produced is small. The disadvantage is obvious, electromagnetic interference (EMI) and more serious.
(2) switching frequency
LED power electromagnetic compatibility problem is generally the power switch circuit. The switch circuit is one of the main sources of interference switching power supply.
LED drive power switching circuit is the core of the switching circuit consists of switches and high-frequency transformer. it produces du / dt has a relatively large pulse, wide-band and rich harmonics.
The main reason is the high-frequency pulse interference: load switch high-frequency transformer primary coil is inductive load.
Figure 1: Switching spikes generated
Turned the moment, the primary coil produces a large inrush current, and the emergence of high surge peak voltage across the primary winding; opened and the moment due to the leakage flux of the primary coil, resulting in no part of the energy transfer from the primary coil to two secondary coil, with a damped oscillation circuit formed spikes superimposed on the turn-off voltage, forming a turn-off voltage spike.
Produce more high-frequency pulse emission, the periodic signal to produce more emissions. In the LED power supply system, the switching circuit generates current spikes, and when the load current changes will generate a current spike. This is one of the sources of electromagnetic interference.
(3) Ground
In all EMC title, the main problem is caused by improper grounding. There are three signal ground methods: single-point, multi-point and mixing. When the switch circuit frequencies below 1MHz, use a single-point ground method, but not suitable for high frequency; in high frequency applications, preferably multi-point grounding. The ground is mixed with a low-single-point ground and the high frequency multi-point grounding method. Ground layout is the key, high-frequency digital circuits and grounding circuit low-level analog circuits do not mix.
(4) PCB design
Appropriate printed circuit board (PCB) routing is essential to prevent EMI.
Reset circuit design (5) Smart LED power supply
In the LED power supply, LED power supply with a lot of intelligent MCU control, and some control of single LED power switching circuit duty cycle, SCM watchdog system running the entire LED power plays a particularly important role, since All sources of interference can not be all it would be a valid defense barrier which provides error correction or addition to, once entered the CPU interfere with the normal operation of the program, then reset the system combines software action. Commonly used to reset the system has the following two:
① external reset system. External "watchdog" circuit can also be used to design their own special "watchdog" to build chips. Thus, if the system program stuck in an endless loop, and the loop has happened to "feed the dog" signal, then the reset circuit can not achieve its proper function of.
② Now more and more LED power supplies have reset the system on its own slice, so users can easily use its internal reset timer, but some intelligent LED power supply control circuit reset command is too simple, so there will still be an infinite loop as described above as "feed the dog" instruction to lose monitoring role.
Hardware Processing Method II of interference measures
To solve the problem of electromagnetic interference LED drive power is available from the following aspects:
1. reduce the switching noise on the power itself
① soft-switching technology: increase the inductance and capacitance element in the original hard switching circuit, the resonant inductor and capacitor to reduce switching process of du / dt and di / dt, so that drops the voltage switching device opened before the current up, down prior to the current rise in voltage, voltage and current to eliminate the overlap time or off.
② switching frequency modulation technique: by modulating the switching frequency fc, the focus fc and its harmonics 2fc, energy 3fc ... spread on the band around them, in order to reduce EMI amplitude of each frequency point.
Select ③ Component: Choose easy to produce noise, easy conducted and radiated noise components. Typically particular note is that the choice of diodes and transformer windings class components. Reverse recovery current, shorter recovery time fast recovery diode is ideally suited for high-frequency switching power supply rectification section.
④ rational use of EMI filters: One of the main purposes of the EMI filter, a power grid noise is electromagnetic interference, radio frequency interference it belongs (RFI), which conducted noise spectrum is substantially 10KHz ~ 30MHz, up to 150MHz.
In general, differential mode interference amplitude, low frequency interference caused by the smaller; common mode interference amplitude, high frequency, can also produce radiation through the wire, the larger the interference caused. To weaken conducted interference, the most effective way is to install the EMI filter input and output switching power supply circuit.
LED power generally use a simple single-stage EMI filters, including common mode choke and filter capacitor.
Figure 2 is a conventional LED power filter, L, C1 and C2 are used to filter out common mode interference, C3 and C4 series mode interference filtered out. When the common-mode interference occurs due to the same magnetic flux direction in the two coils L, after coupling the total inductance increases rapidly, so the common mode signal exhibits a large inductive reactance, so that is not easy to pass, so called common mode choke coil. Its two coils are wound on a ferrite toroid low loss, high magnetic permeability. R is the discharge resistance can be accumulated charge on C3 vent let go, to avoid charge accumulation affect the filter characteristic, but also to power after the end of the power line L, N is not charged, to ensure the safety of use.
Figure 2 conventional LED power filter
⑤ EMI filter can effectively suppress electromagnetic interference Switching power adapter
FIG. 3 is a curve without EMI filter switch on the power adapter 0.15MHz ~ 30MHz conduction noise waveform.
Curve b is a waveform added after EMI filters, electromagnetic interference can be attenuated 50 db (Uv) ~ 70 db (uV). Clearly, better EMI filter insertion.
Front and rear transmission electromagnetic interference filter was added to the noise voltage across the load, respectively U1 and U2, the formula is 20lgU1 / U2.
Expressed in decibels dB insertion loss, the greater the decibel value, indicating a stronger ability to suppress noise.
Add loss measuring circuit shown in FIG. e is the noise signal generator, Zi is the internal impedance of the signal source, ZL is the load impedance, and generally 50 ohms. Optional noise frequency range 10KHz ~ 30MHz. First were measured at different frequencies of noise around U1 join EMI filter pressure drop across the load, U2, and then into the formula 20lgU1 / U2 calculated insertion loss values for each frequency, and finally export insertion loss curves.
Figure 3: adherence EMI filter before and after
2. cut off the route of transmission of the interfering signal
① power line disturbances can use the filter to remove the power cord. A reasonable and effective switching power supply EMI filter should supply line differential mode and common mode interference has a strong inhibitory effect.
② improve the electromagnetic compatibility design of PCB
PCB is LED power supply system support components and devices, which provide electrical components and connections between devices. With the rapid development of electronic technology, PCB density is increasing. PCB design quality great influence on electromagnetic compatibility LED power supply.
Practice has proved that, even if the circuit schematic design right, printed circuit board design properly, will adversely affect the reliability of the LED power supply.
PCB anti-jamming design includes PCB layout, wiring and grounding, which aims to reduce crosstalk and electromagnetic radiation PCB on the PCB circuit.
Also, the general transformer electromagnetic interference caused by the hum frequency is generally about 50HZ, improper wiring and grounding hum due to the multiplier effect of the frequency of the rectifier circuit is about 100HZ, carefully distinguish still perceptible.
The correct wiring method is to select the main filter capacitor pin as a centralized ground, strong and weak signal ground strictly separated, are summarized in the total ground.
Therefore, in the design of printed circuit boards, we should pay attention to the correct way to comply with the general principles of PCB design, and shall conform to the requirements of anti-jamming design.
3. The enhanced anti-jamming capability disturbed body
In the LED power supply system input / output is also the source of interference conductive lines, and receiving radio frequency interference signal source Pick up, we generally take effective measures designed to:
① adopt the necessary common-mode / differential mode suppression circuit, but also to take certain filtering and anti-electromagnetic interference shielding measures to reduce feed intake.
② take as much as possible under various conditions permit isolation measures (such as magnetic or optical isolation isolation), thereby blocking the spread of interference.
③ lightning protection measures
LED power system for outdoor use or exclusion from the introduction of indoor outdoor power lines, signal lines, to consider the system of lightning problems. Commonly used anti-lightning devices include: gas discharge tube, TVS (Transient Voltage Suppression) and the like. Gas discharge tube when the power supply voltage is greater than a certain value, typically tens or hundreds of V V, gas breakdown discharge, the strong impulse power line lead into the earth. TVS can be seen as two parallel and opposite direction Zener diode when the voltage across the conduction above a certain value. Which can be characterized by a transient current is hundreds of thousands of A's.
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